1. Technical Field
The present invention relates to a surface acoustic wave oscillator including surface acoustic wave elements having different resonance frequencies and a cross-coupled circuit that differentially connects those elements and to a method of varying the frequency thereof.
2. Related Art
In mobile communication devices communication is performed by, for example, switching multiple channels within the range of the allocated frequency bandwidth. Therefore, it is necessary to perform switching to the designated channel (frequency) at a control channel. Therefore, a synthesizer including a Phase Locked Loop (PLL) circuit has been used. As this PLL circuit, a voltage controlled oscillator (VCO) has been widely used, in which the oscillating frequency is switched by applying a control voltage. As a conventional example of the VCO in which the oscillating frequency is switched by applying a control voltage, a known method changes the electrostatic capacitance by applying a control voltage to a variable capacitance diode, thereby shifting a resonance point of the surface acoustic wave (SAW) elements.
However, when switching channels within the used frequency bandwidth, a high SN ratio (ratio of signal power and noise power) and a high CN ratio (ratio of carrier power and noise power) are demanded for the entire range of this frequency bandwidth. However, with a conventional VCO, it is rather difficult to keep the SN ratio and the CN ratio high with respect to all of the frequency bandwidth, and when the amount of frequency change becomes large, the SN ratio and the CN ratio becomes disadvantageously lower. On the other hand, when an attempt is made to keep the SN ratio and the CN ratio high, the variable range of the frequency becomes disadvantageously narrower. To solve these problems, there is a method of allocating each half of the frequency bandwidth by using two VCOs. However, the use of two VCOs contradicts the miniaturization of communication devices and therefore cannot be considered as a practicable solution.